In a cellular communication system, one base station performs radio communication with a plurality of communication terminals simultaneously, and therefore, as demand has increased in recent years, so has the need for higher transmission efficiency.
One technology for increasing the transmission efficiency of a downlink from a base station to a communication terminal is HDR (High Data Rate). HDR is a method whereby scheduling for assigning communication resources to communication terminals is performed by time division, and data transmission efficiency is further improved by setting the transmission rate for each communication terminal according to the communication quality. This method is suitable for connection via the Internet and the like.
The operations performed by a base station and communication terminals in order to set the transmission rate in HDR will be described below using FIG. 1. In FIG. 1, a base station 11 is currently performing communication with communication terminals 12 to 14.
First, the base station 11 transmits a pilot signal to each of communication terminals 12 to 14. Each of communication terminals 12 to 14 estimates the communication quality according to the SIR (Signal to Interference Ratio) of the received pilot signal, etc., and finds a transmission rate at which communication with the base station is possible. Then, based on the transmission rate at which communication is possible, each of communication terminals 12 to 14 selects a communication mode, which is a combination of packet length, error correction, and modulation method, and transmits a signal indicating the communication mode to the base station 11.
Based on the communication mode selected by each of communication terminals 12 to 14, the base station 11 performs scheduling, sets a transmission rate for each communication terminal, and sends a signal to each of communication terminals 12 to 14 via a control channel indicating communication resource assignment to each communication terminal.
The base station 11 then transmits only data for the relevant communication terminal in its assigned time. For example, if time t1 has been assigned to communication terminal 12, the base station 11 transmits data for communication terminal 12 in time t1, and does not transmit to communication terminals 13 and 14.
In this way, data transmission efficiency has conventionally been increased for the overall system by setting a transmission rate for each communication terminal according to the communication quality by means of HDR.
Here, the communication quality of some bands may degrade due to frequency selective fading. Also, the band portion for which communication quality degrades differs for each communication terminal.
In FIG. 2, for example, communication terminal 12 has good communication quality on the high-frequency side but poor communication quality on the low-frequency side, while communication terminal 13 has good communication quality on the low-frequency side but poor communication quality on the high-frequency side, and communication terminal 14 also has good communication quality on the low-frequency side but poor communication quality on the high-frequency side.
However, as the above-described conventional base station and communication terminals perform communication using the entire band, a problem is that communication quality degrades and transmission efficiency falls in a band affected by frequency selective fading.